Topic: '78 CB550k - cyl4 not firing. What to test?

[toomanybikes]

Resurrecting a 550k that sat for years. 10k miles, generally well kept, and unmolested. Everything is stock from what I've seen.

During yesterday's maiden voyage with new tires, chain, sprockets I noticed the engine performance was very weak, especially when opening the throttle. Get home and find the exhaust for cyl4 is warm/hot, but not wicked hot like the others. A few more tests after letting it cool indicates that cyl4 isn't firing.

Looking for guidance on what to check next given that I've already:
- replaced battery
- cleaned and synced carbs with new valves and gaskets. I'm sure there is gas getting thru the main jet when the throttle is open since these are not CV carbs with torn diaphragms
- checked and adjusted valve clearances. Now in spec.
- checked compression, each cyl at 130+ psi
- swapped plugs among the cylinders. no diff in behavior
- pulled the plug for cyl4, tested for spark by holding it against engine while turning crank. there is some spark. Would that rule out a bad cable or coil?
- checked points gaps. The RH gaps is .014, while the left one  is .017-.018. I know that's out of spec and will adjust it, but why would that affect only one cylinder?
- i have not adjusted the timing plate or checked the coils yet. Will do that today. But wouldn't a bad coil also affect another cylinder?
- the points and ignition looks to be in good condition (see pic), but I'm not an expert

Is there anything else I should check or something that is easily replaced?

[OldSchool_IsCool]

Run ‘er in a dark garage or at night.  Look for any arcing along the #4 wire.  It could be that the spark is being lost due to a crack in the insulation.

[69cb750]

All it needs is fuel, compression, ignition.
If plug is black you have ignition trouble, if plug is clean you have lack of fuel, replace the idle jet, spay idle passage with carb cleaner.

[toomanybikes]

All it needs is fuel, compression, ignition.
If plug is black you have ignition trouble, if plug is clean you have lack of fuel, replace the idle jet, spay idle passage with carb cleaner.

I'm >99% sure it's ignition trouble. that's what I'm trying to narrow down.

[seanbarney41]

All it needs is fuel, compression, ignition.
If plug is black you have ignition trouble, if plug is clean you have lack of fuel, replace the idle jet, spay idle passage with carb cleaner.

I'm >99% sure it's ignition trouble. that's what I'm trying to narrow down.

that is interesting because 99 percent of the time with your symptoms, it is the carb...probably a pilot jet that needs to be re-cleaned.  All it takes to block one of these tiny jets is a drop of water.

[toomanybikes]

All it needs is fuel, compression, ignition.
If plug is black you have ignition trouble, if plug is clean you have lack of fuel, replace the idle jet, spay idle passage with carb cleaner.

I'm >99% sure it's ignition trouble. that's what I'm trying to narrow down.

that is interesting because 99 percent of the time with your symptoms, it is the carb...probably a pilot jet that needs to be re-cleaned.  All it takes to block one of these tiny jets is a drop of water.

yes but ... I've thoroughly cleaned the carbs and the unfiring cyl occurs even after a 10 mile ride with throttle open well past the idle circuit. So even if the idle circuit were clogged, the regular circuit would be delivering gas thru the main jet, which is easy to visually verify being unclogged.

[seanbarney41]

All it needs is fuel, compression, ignition.
If plug is black you have ignition trouble, if plug is clean you have lack of fuel, replace the idle jet, spay idle passage with carb cleaner.

I'm >99% sure it's ignition trouble. that's what I'm trying to narrow down.

that is interesting because 99 percent of the time with your symptoms, it is the carb...probably a pilot jet that needs to be re-cleaned.  All it takes to block one of these tiny jets is a drop of water.

yes but ... I've thoroughly cleaned the carbs and the unfiring cyl occurs even after a 10 mile ride with throttle open well past the idle circuit. So even if the idle circuit were clogged, the regular circuit would be delivering gas thru the main jet, which is easy to visually verify being unclogged.

I would think the same thing, but from my experiences it doesn't always work this way.  It is quite possible you never even opened the main circuit in the course of a short, aborted ride.  Another possibility is float bowl fuel level too low to pick up any fuel.  Fortunately, we know it is cylinder 4, so you have easier options than pulling the whole carb rack.  You could do a clear tube test and if that does not reveal any gremlins, use a shorty screw driver to drop number 4 bowl and clean out the pilot jet.  Also I would get those points set within spec first.

[bryanj]

Most of the time you need to pull out the pressed in pilot jet and thouroughly clean.

[JeffK]

Just my opinion.

If you have spark and your pulling air through the #4 cylinder then you have a carb issue. Your other three cylinders are firing so it is not a timing issue.

Take some time and buy some carb rebuild kits and rebuild them. Be sure you check the float levels during the rebuild. Be sure and  "bench-level" the needles. There are really good videos on the net to help you with this and it is really easy.

Be sure and check for vacuum leaks in #4 also.

[toomanybikes]

Just my opinion.

If you have spark and your pulling air through the #4 cylinder then you have a carb issue. Your other three cylinders are firing so it is not a timing issue.

Take some time and buy some carb rebuild kits and rebuild them. Be sure you check the float levels during the rebuild. Be sure and  "bench-level" the needles. There are really good videos on the net to help you with this and it is really easy.

Be sure and check for vacuum leaks in #4 also.

I'll double check the carbs, initially on the bike with compressed air before pulling them off again. I have already checked the float level of carb4 and it's right at spec while mounted on the bike. Tested by slower lifting float until the valve plunger begins to compress.

Would be happy if compressed air solves it because I'm better with carbs than with electricals.

[Deltarider]

Maybe I missed it, but is #4 plugcap in good order and well connected?

Run ‘er in a dark garage or at night.  Look for any arcing along the #4 wire.  It could be that the spark is being lost due to a crack in the insulation.

I second OldSchool. This test is easily done. The CB500/550s were notorious for arcing sooner or later and Honda tried all kinds of plugcaps to address this. On the other hand the slow jets in the PD carbs were notorious too. Are vent tubes (and/or holes) open?

[toomanybikes]

Maybe I missed it, but is #4 plugcap in good order and well connected?

Run ‘er in a dark garage or at night.  Look for any arcing along the #4 wire.  It could be that the spark is being lost due to a crack in the insulation.

I second OldSchool. This test is easily done. The CB500/550s were notorious for arcing sooner or later and Honda tried all kinds of plugcaps to address this. On the other hand the slow jets in the PD carbs were notorious too. Are vent tubes (and/or holes) open?

Interesting thought on the vent tube and one I forgot to mention. The overflow pipe of the float bowl on carb4 has a slow leak so I currently have a rubber stopper covering it rather than an open ended hose. Since the stopper is not allowing it to vent it may be affecting the fueling. Wicked easy to check and needed to be fixed anyway.

I already checked the points gaps and timing plate, which didn't fix the issue. Back to the garage to pull off that pesky stopper!

[bryanj]

Vents are seperate but if fuel is dripping from overflow its a VERY bad idea to plug it, you need to fix the problem!

[toomanybikes]

Vents are seperate but if fuel is dripping from overflow its a VERY bad idea to plug it, you need to fix the problem!

Agreed. My first fix with shrink wrap didn't work because the available wrap wasn't compatible with fuel so it unshrunk. Need to get fuel compatible shrink wrap or come up with another solution. or just buy a replacement float bowl.

BTW - to test the cap/wire from cyl1 can I swap the cap and wire still connected between 1 & 4? The PO has some shrink wrap on plug1 so I cant easily remove it. Since there is a "wasted" spark will the easy swap over work?

[OldSchool_IsCool]

1 to 4 swap will work if there is enough wire length

[bryanj]

Dont you think that if some idiot wrapped shrinkwrap round a plug cap that there is a problem!

[toomanybikes]

UPDATE - Well ... chalk another one up for the really old timers. It was the carbs.

Swapping plug wires 1 & 4 wasn't possible due to wire length, so I concentrated on the carbs. Did a mini "on the bike" carb check by pulling all the float bowls, floats, valves, and main jet/emulsifier tubes. Unfortunately those pressed in slow jets weren't coming out.

Turns out the main jet/emulsifier tube on carb4 wasn't clear. Don't know how it could have gotten gummed up again, but it was so back into the ultrasonic it went.

A few of the float heights also needed adjustment, so it was probably running a little lean.

Now she's firing on all four and cyl4 exhaust is getting hot. Still not as hot as the other three, so maybe the mixture screws may still need a little tweaking. They are all currently at 1.5 turns out.

Now to find a fix for that leaky float bowl#4.

Thanks all for the quick help!